1. Field of the Invention
The present invention relates to a method for manufacturing a superconducting device, and more specifically to a novel method for manufacturing a superconducting device including a tunnel junction composed of a pair of superconducting electrode layers composed of oxide superconductor materials, and an insulating layer sandwiched between the pair of superconducting electrode layers
2. Description of Related Art
Superconducting materials known in the prior art had become a superconductor only at an extremely low temperature not higher than a liquid helium temperature, and therefore, it had not been considered to practically utilize the superconducting material. However, since it was reported in 1986 that compound oxide sintered material such as (La, Ba).sub.2 CuO.sub.4 or (La, Sr).sub.2 CuO.sub.4 are a superconductor material having a high critical temperature (Tc), it has been confirmed from one to another that a compound oxide such as a Y--Ba--Cu--O type compound oxide or a Bi--Ca--Sr--Cu--O type compound oxide shows a superconducting characteristics at an extremely high temperature. These materials showing the superconducting characteristics at the high temperature permit to use an inexpensive liquid nitrogen for a cooling medium, and therefore, application of superconductor technique has abruptly been put under study as an actual matter.
A Josephson device is one of elementary electronic devices utilizing the superconducting phenomenon. The Josephson device is constituted of a weak link or junction between a pair of superconducting electrodes, and has various unique characteristics, for example, a DC Josephson effect which is said to be due to a tunnel effect of cooper pairs, an AC Josephson effect showing a discrete voltage/current characteristics, etc.
On the other hand, the oxide superconducting materials were obtained in the form of a sintered body produced by a powder metallurgy process at the beginning of research and development, but the sintered oxide superconducting materials could not have a good result, particularly in a critical current density. Lastly, a method for manufacturing the oxide superconducting materials in the form of a thin film are widely studied. One typical example of the Josephson devices formed of the oxide superconducting thin films is a tunnel type Josephson device including the superconducting tunnel junction.
The most typical construction of a tunnel junction type Josephson device includes a first superconducting layer formed of an oxide superconductor thin film on a substrate, an insulator layer formed on the first superconducting layer, and a second superconducting layer formed of an oxide superconductor thin film on the insulator layer. Here, the insulator layer is extremely thin, and actually is formed of an insulator thin film having a thickness of not more than 20 .ANG..
In the above mentioned tunnel junction type Josephson device, formation of the insulator layer requires a greatly high level of technique. Namely, the insulator layer is required to have the thickness of not more than 20 .ANG. as mentioned above, but it is very difficult to form this extremely thin insulator film without pinholes, on an oxide superconductor material layer. If this extremely thin insulator layer has pinholes, the first and second superconducting layers short-circuit to each other, so that the device no longer exerts its function. For this reason, it has been said to be difficult to actually form a tunnel type junction.